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Hey MyFirstGrow, welcome to RUI!
Are you PH adjusting your water? It's always the first question I ask new growers.
Not fertilizer burn or heat stress, it looks like some kind of nute deficiency, often caused by imbalanced PH.
Do you have a PH meter?
What kind of soil are you using?
Does the soil come premixed with nutes (like Miracle-Gro)?
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The first thing we have to to is balance the PH of your root ball and wash out ALL of the nutes - resetting your root ball and starting from zero. This is easily done by slowly pouring distilled water into your cup - 3x the volume of you're cup - raising the cup and letting the flow-though go down the sink. The flush should be done sooner than later, distilled water can be purchased in any pharmacy ($2.00 a gallon) in the eye care section.
Your root ball PH is now 7, very close to the ideal 6.3-6.8 ph. Leave the plant be under a good but not overpowering light -> until the cup dries up. We have to let the breathing roots dry out or they will rot. The dry/soak cycle also pushes the roots to grow, both in length and diameter, this improves everything.
After your cup is dry mix your nutes in distilled water and PH balance after adding nutes.
The damaged leaves are going to die, but that doesn't matter - they're too low on the plant to matter. I'd snip all leaves that low before going into flower. You should be fine, keep us updated and ask for clarification if the explanation isn't understandable.
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The Ecotestr ph 2 is the best tester I've used - very fast and resilient, relatively inexpensive.
http://www.4oakton.com/proddetail.asp?parent=2&prod=354&seq=1&Totrec=21
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